1. Field of the Invention
The present invention relates generally to a capacitor discharge type ignition apparatus for an internal combustion engine (hereinafter also referred to simply as the engine). More particularly, the invention is concerned with an ignition apparatus for the engine which can ensure an adequately high charge voltage of a capacitor even in a high-speed operation of the engine to thereby stabilize ignition control.
2. Description of the Related Art
Heretofore, there is known such a capacitor discharge type ignition (CDI) apparatus for internal combustion engines in which a boosted voltage charged previously in a capacitor is discharged through a primary winding of an ignition coil, to thereby generate a spark discharge at a spark plug. This capacitor discharge type ignition apparatus is advantageous in that the ignition for firing a fuel mixture with an associated engine cylinder can positively be ensured even when the spark plug is in a stained state or when a battery of a small capacity is employed, because this type ignition apparatus features a short discharge time and a steep rise-up of the discharge current.
As a booster circuit for charging the capacitor of the ignition apparatus of the type mentioned above, a DC-DC converter is employed which includes a power transistor for turning on and off a booster coil in a repetitive manner.
FIG. 5 shows a circuit configuration of a capacitor discharge type ignition apparatus known heretofore in which a DC-DC converter is employed. Referring to the figure, a reference numeral 1 denotes a battery serving as a power supply source for supplying electric energy to the whole apparatus. Connected to the battery 1 is a DC-DC converter 2 which is comprised of a transformer type booster coil 3, a power transistor 4, a resistor 5, and diodes 6 and 7, which are connected in a manner as shown in the figure, to constitute a booster circuit for generating a boosted voltage by repeatedly turning on and off the power transistor 4 connected to the battery 1.
More specifically, the booster coil 3 includes a primary winding 3a having one end connected to the battery and the other end connected to a collector of the power transistor 4, a secondary winding 3b for outputting a boosted voltage from one end thereof through the diode 7 and a switching secondary winding 3c for turning on and off the power transistor 4 at a high frequency on the order of 20 kHz. Applied to a base electrode of the power transistor 4 together with a source voltage from the battery 1 via the base resistor 5 is an output voltage from the switching secondary winding 3c of the transformer type booster coil 3. The diode 6 serves to prevent a reverse bias voltage from being applied to the base of the power transistor 4.
A capacitor 8 is connected to an output terminal of the DC-DC converter 2 to be thereby charged with the boosted voltage which is outputted from the DC-DC converter 2. An ignition coil 9 has a primary winding 9a connected to the capacitor 8 and a secondary winding 9b connected to a spark plug 10 which serves to fire a fuel mixture within an associated engine cylinder. A fly-wheel diode 11 is connected across the primary winding 9a of the ignition coil 9.
A thyristor 12 is inserted in a discharge path of the capacitor 8 which includes the primary winding 9a of the ignition coil 9. This thyristor 12 is turned on in response to a trigger signal (described hereinafter) applied to the gate thereof. A resistor 13 of about 1 k.OMEGA. is connected between the gate and the cathode of the thyristor 12 and serves for determining a current value, i.e., a hold current I.sub.H in the off-state of the thyristor 12.
At this juncture, it should also be mentioned that a voltage detecting circuit is provided for detecting attainment of a capacitor charge voltage d to a predetermined voltage level, to thereby interrupt operation of the DC-DC converter 2, although illustration of such voltage detecting circuit is omitted.
An electromagnetic pickup or sensor 14 is disposed in opposition to an output shaft (not shown) of the engine for generating a ignition timing signal a at every predetermined crank angle. The ignition timing signal a is rectified by a diode 15 so that it assumes a positive polarity after the rectification. A resistor 16 serves to output the rectified ignition timing signal as the aforementioned trigger signal which is applied to the gate of the thyristor 12. As can easily be understood, the electromagnetic pickup 14, the diode 15 and the resistor 16 cooperate to constitute a trigger circuit for turning on the thyristor 12 in synchronism with the ignition timing signal generated at an ignition timing required for the desired engine operation.
A monostable multivibrator 17 is provided for generating a pulse c for stopping the voltage boost operation in response to the gate voltage b of the thyristor 12 derived from the trigger signal. More specifically, the monostable multivibrator 17 is so designed as to generate the boosting operation stop pulse c which has a duration extended by a turn-off period t1 taken for the thyristor 12 to become completely nonconducting upon falling of the gate voltage b. A transistor 18 which is turned on in response to the boosting operation stop pulse c is connected between the resistor 5 of the DC-DC converter 2 and the ground.
FIG. 6 is a circuit diagram showing, by way of example, a configuration of the monostable multivibrator 17. Referring to the figure, this monostable multivibrator 17 includes an input terminal 21 to which the gate voltage b is applied, an output terminal 22 for outputting the boosting operation stop pulse c, an emitter-grounded transistor 23 having a base connected to the input terminal 21, an emitter-grounded transistor 24 having a collector connected to the output terminal 22, a capacitor 25 inserted between the ground and a junction between the collector of the transistor 23 and the base of the transistor 24, and a resistor 26 inserted between the capacitor 25 and the battery.
In the monostable multivibrator shown in FIG. 6, when the gate voltage b is at a low level "L", the transistor 23 is in the off-state with the base potential of the transistor 24 being high "H". Consequently, the transistor 24 is also in the off-state. The boosting operation stop pulse c is thus at the low level "L". Assuming that the gate voltage b rises up to a high level "H", starting from the state mentioned above, the transistor 23 is turned on, to thereby render the base potential of the transistor 24 to be low "L". Accordingly, the transistor 24 is turned off, whereupon the boosting operation stop pulse c of the level "H" is outputted from the output terminal 22. At this time point, the electric charge stored in the capacitor 25 is drained to the ground through the collector-emitter path of the transistor 23. Subsequently, the gate voltage b falls to the level "L", and the transistor 23 is turned off. However, so long as the capacitor 25 is electrically charged, the base potential of the transistor 24 is inhibited from becoming high "H". After lapse of the charging time t1, the transistor 24 is turned on, as a result of which the boosting operation stop pulse c assumes the low level "L". The charging time t1 is previously so set as to coincide with the turn-off timing of the thyristor 12 by selecting appropriately the values of the capacitor 25 and the resistor 26.
FIG. 7 is a waveform diagram illustrating waveforms of the signals generated in the apparatus shown in FIG. 5. The signals a, c and d have already been mentioned. A waveform e is of a current which is discharged from the secondary winding 3b of the booster coil 3 when the thyristor 12 is turned on.
Now, by reference to FIG. 7, description will be directed to the operation of the known ignition apparatus shown in FIG. 6.
Ordinarily, the capacitor 8 is charged with a boosted voltage supplied from the DC-DC converter 2. When the electromagnetic pickup 14 generates the ignition timing signal a at a predetermined ignition timing determined by the engine operation as desired, starting from the above-mentioned state, the ignition timing signal a of positive polarity is applied to the gate of the thyristor 12 as the trigger signal by way of the resistor 16.
Thus, the thyristor 12 is turned on, whereby the charge voltage d of the capacitor 8 is instantaneously discharged through a discharge path including the primary winding 9a of the ignition coil 9 and the thyristor 12. Usually, the time taken for this discharge is on the order of 20 .mu. sec. Consequently, a high voltage is induced in the secondary winding 9b of the ignition coil 9, resulting in that a spark discharge takes place at the spark plug 10 to fire or ignite the fuel mixture within the associated engine cylinder at the desired ignition timing.
In the conducting state of the thyristor 12, the gate voltage b is continuously outputted. When the conducting current, i.e., the discharge current of the capacitor 8 becomes lower than the hold current I.sub.H, the thyristor 12 is instantaneously turned off (i.e., becomes nonconducting).
On the other hand, when the trigger signal is outputted via the resistor 16, the monostable multivibrator 17 produces the boosting operation stop pulse c to thereby turn on the transistor 18, whereby the base current of the power transistor 4 is broken to stop the operation of the DC-DC converter 2. Subsequently, the thyristor 12 is turned off and the gate voltage b thus falls. After lapse of the turn-off period t1 of the thyristor 12, the boosting operation stop pulse c falls. Thus, operation of the DC-DC converter 2 is restarted to charge again the capacitor 8. In this manner, only after the thyristor 12 have assumed the intrinsically nonconducting state, the boosting oscillation takes place, whereby loss in the oscillation energy can be prevented.
More specifically, when the battery voltage is applied to the base of the power transistor 4 from the battery via the resistor 5, the power transistor 4 is turned on to allow the current to flow through the primary winding 3a of the booster coil 3. At that time, voltages of opposite polarities are induced in the secondary windings 3b and 3c, respectively. In that case, an alternating voltage of ca. 20 kHz is generated across the switching secondary winding 3c, whereby the power transistor 4 is turned on and off at a high frequency, to interrupt the conduction of the primary current. Thus, a boosted voltage is generated from one end of the secondary winding 3b to charge the capacitor 8.
Incidentally, the diode 6 functions to cut the component of negative polarity from the voltage generated across the switching secondary winding 3c to prevent a reverse bias from being applied to the base of the power transistor 4. Protection of the power transistor 4 against the reverse bias can thus be ensured.
When the charge voltage d of the capacitor 8 has attained a predetermined voltage due to repetition of the charging operation, the voltage detecting circuit (not shown) interrupts the operation of the DC-DC converter 2 to thereby protect the capacitor 8 from being overcharged.
In the normal operation, the trigger signal for the thyristor 12 is generated after the operation of the DC-DC converter 2 is stopped by the voltage detecting circuit. Thus, the DC-DC converter 2 is restarted upon lapse of the turn-off period after the discharge of the capacitor voltage d. However, in the high-speed operation of the engine, the time interval at which the ignition timing signal a is generated becomes shorter. Consequently, the trigger signal is generated during operation of the DC-DC converter 2, i.e., during electric conduction of the booster coil 3.
When the situation mentioned above takes place, the electromagnetic energy of the booster coil 3 which has usually a high impedance is discharged as a discharge current e (refer to FIG. 5). As a result of this, the conducting period of the thyristor 12 is correspondingly extended. In this conjunction, it is noted that since the gate voltage b continues to flow until the booster coil discharge current e diminishes below the hold current I.sub.H.
For the reason described above, when the boosting operation stop pulse c is extended, as shown in FIG. 5, the DC-DC converter 2 remains inoperative during an correspondingly elongated period, involving a time lag in the timing for the restart of the DC-DC converter, which means that no adequate charge voltage d of the capacitor 8 can be obtained in the succeeding boosting oscillation cycle. In particular, in the high-speed engine operation, the trigger signal derived from the ignition timing signal a is generated at a short periodic interval, whereby the charge voltage d is further decreased to prevent the spark discharge at the spark plug 10 from occurring at a desired timing.
As is apparent from the foregoing discussion, in the known ignition apparatus for the internal combustion engine, the hold current I.sub.H of the thyristor 12 is determined by the resistor 13. Further, the ignition timing signal a generated in every ignition cycle is utilized as the trigger signal for discharging the capacitor 8. Accordingly, when the electromagnetic energy stored in the booster coil 3 of the DC-DC converter 2 is released through the thyristor 12 in the high-speed engine operation, the thyristor 12 continues to remain in the conducting state for an elongated period, resulting in that the DC-DC converter is caused to remain in the interrupted state for a longer time than the period actually required. Consequently, the succeeding charging period is correspondingly shortened to render it impossible to charge the capacitor 8 adequately, providing an obstacle in realizing the stable ignition process. In particular, in the high-speed engine operation state, the trigger signal based on the ignition timing signal can not be generated at a shorter time interval, as a result of which the charge voltage d becomes further lowered, preventing the firing of the spark plug at a predetermined timing.